Glass for windows and automobile windshields or the like should be clear and substantially free from significant surface imperfections which could interfere with the transmission of light or affect the appearance of the glass. One defect in glass which can render the glass unacceptable for such applications is the presence of a slight haze or discoloration on a surface of the glass. Surface hazing which occurs at normal handling temperatures (as opposed to hazes which develop at high temperatures during tempering or annealing operations) is referred to herein as "staining".
Staining is a major problem in the glass industry. Although panes of glass which exhibit some staining may be acceptable for some applications, in most circumstances any appreciable staining can render the glass virtually useless. For example, window units and windshields are frequently provided with heat-reflective metal/metal oxide coatings. When such a coating is applied to a glass surface exhibiting stain, the resultant coated article has a hazy, uneven appearance which can render the coated glass unsalable.
Although one may postulate possible chemical processes by which staining occurs, the exact mechanism of staining of glass surfaces is not understood. There appears to be a relationship between humidity and staining. Glass manufactured and stored in a hot, humid environment where the ambient air has a high moisture content is more likely to show staining than glass manufactured on a drier day or stored in a drier climate. Accordingly, staining may perhaps be attributable in part to the effect of moisture on glass surfaces. However, the precise mechanism of staining does not appear to have been satisfactorily determined.
Virtually all of the glass being manufactured today for use in windows and the like is float glass. The float glass manufacturing process is well known. Briefly, molten glass is deposited on a bath of molten tin which may include some minor additions of other metals. The molten glass is drawn across the tin bath in a ribbon by pulling on the glass ribbon as it exits the tin bath and enters a cooling or annealing area. The surface tension of the glass on the molten tin produces a smooth finish on the glass which was obtainable in prior methods only via extensive grinding and polishing. As the glass cools on the tin bath, the underside of the glass is in direct contact with the tin and it is believed that some tin becomes incorporated in the bottom surface of the glass.
The temperature in the tin bath is profiled along its length so that the glass will cool and harden somewhat as it traverses the bath. When the glass reaches the end of the tin bath it needs to be stiff enough to be handled as a continuous ribbon, yet it must be soft enough to permit it to be lifted up over a lip of the tin bath and onto rollers for conveyance through an annealing lehr. The glass is cooled in the annealing lehr in a controlled fashion to avoid undesirable thermal stresses in the glass. During the treatment of the glass ribbon in the annealing lehr it is passed over a series of rollers which support the underside of the ribbon.
As noted above, when the glass first leaves the tin bath and enters the annealing lehr, it must be soft enough to permit it to be lifted up over the lip of the tin bath. This relatively soft glass then rests on a series of rollers as it is transported through the annealing lehr. Any surface imperfections in these rollers can lead to a marred surface if the glass remains untreated. In most applications, though, a smooth finish on the glass is needed. Also, when the glass is relatively soft it can tend to stick to the surfaces of the rollers, further marring the finish of the glass.
Due to imperfections in the undersurface of float glass resulting from roller contact, metal/metal oxide coatings of the type commonly used in heat reflective windows and windshields are applied preferentially to the upper surface of the glass, i.e. that surface which was not in contact with the rollers during manufacture of the glass sheets. Such metal oxide coatings are generally known to the art and may be applied, for example, by magnetron sputtering techniques. When a particular batch of glass is found to exhibit excessive staining, some commercial manufacturers have tried to ameliorate this problem by applying metal oxide coatings to the underside of the glass which had been in contact with the rollers.
When the undersurface of the float glass is coated with a metal/metal oxide coating in this manner, it has been found that the undersurface virtually never exhibits staining. It has heretofore been believed that the incorporation of tin in this undersurface, resulting from contact with the molten tin bath, was responsible for the lack of staining. Although underside coating may provide a temporary solution in a production situation where better glass is not available when needed, surface imperfections in the glass surface due to contact with the rollers make coating the underside of the glass on a consistent commercial basis impractical.
U.S. Pat. No. 3,199,966 (the teachings of which are incorporated herein by reference), issued to O'Connell et al, suggests a method which has gained wide-spread acceptance in the art for reducing surface imperfections resulting from contact with the rollers. In this method, sulfur trioxide (SO.sub.3) is applied to the undersurface of the glass before it comes into contact with the rollers. O'Connell et al explain that the application of SO.sub.3 to the bottom of the glass ribbon provides a lubricious coating between the glass and the rollers, allowing the glass to move over the rollers without creating as many surface imperfections due to contact with the rollers. Additionally, O'Connell et al state that the SO.sub.3 reacts with sodium oxide (Na.sub.2 O) in the glass surface to form sodium sulfate (Na.sub.2 SO.sub.4) and that some of this salt is transferred to the rollers, providing a smoother, more uniform surface on the rollers.
Although the process of O'Connell et al has been found to reduce the surface imperfections attributable to contact with the rollers in the annealing lehr, it does not completely eliminate such flaws. The presence of these mechanically generated surface flaws still makes the application of metal/metal oxide coatings to the undersurface of the glass a commercially impractical means of reducing staining losses on a regular basis because the resultant coated articles are likely fraught with visible imperfections. In high quality window products, such imperfections are not commercially acceptable and coated glass exhibiting such flaws is generally deemed to be unacceptable for commercial sale.
Snow, U.S. Pat. No. 3,473,908 (the teachings of which are incorporated herein by reference) outlines an additional advantage of applying SO.sub.3 to the underside of the glass. Snow explains that float glass tends to develop a "white, iridescent haze" when it is heated above temperatures of about 1000.degree. F. for purposes of tempering or reshaping (e.g. to form a windshield from flat float glass). Snow attributes this haze to the conversion of stannous oxide to stannic oxide on the glass undersurface that was in contact with molten tin. Snow surmises that the reaction of SO.sub.3 with sodium oxide in the glass undersurface ties the tin more intimately to the glass network, preventing the stannous oxide from converting to stannic oxide.
Attempts also have been made to reduce staining of the upper side of sheets of float glass. As noted above, the superior stain resistance of the underside of the glass was believed to be due to the intimate contact between the molten tin and the glass surface in the tin bath and a resulting incorporation of tin oxide into the glass surface. No practical means for bringing the upper surface of the glass into contact with molten tin (at least without applying a visible layer of tin or tin oxide to the glass surface) appears to have been developed. Hence, glass manufacturers have had to look to other solutions to reduce stain.
One may attempt to get around this problem by altering the composition of the glass. Ordinary float glass used for windows and the like is most commonly "soda-lime glass". Such glass includes primarily silica (from sand), sodium oxide (usually from soda ash) and calcium oxide (usually from limestone). Minor additions of other constituents may be added to achieve different properties, such as to color the glass.
In order to reduce staining, some manufacturers have added other materials to the glass for the purpose of reducing the susceptibility of the glass to moisture or chemical attack; alumina is one common material added for this purpose. Many such additives, including alumina, tend to be more expensive as raw materials than the common constituents of soda-lime glass and also tend to melt at notably higher temperatures, increasing the cost of melting and making the glass.
Others have attempted to reduce staining of the glass upper surface by treating the upper surface with acids. The most common acid treatment for glass surfaces involves the, application of a powdered adipic acid to the glass at the end of the annealing process. Others have applied adipic acid or maleic acid in aqueous form on the glass. When the glass is washed before applying heat-reflective films, these water-soluble acids are generally washed away as well. Much research and development effort has been devoted to developing specific acid compositions and methods of application to combat staining. The specific acid treatments which are developed are frequently treated as trade secrets by the manufacturers.
Modifying the composition of the glass and applying various acids to the surface does tend to reduce staining, but does not eliminate it. Despite the well recognized problem of staining and the massive attempts by glass manufacturers to eliminate this costly problem, to date there do not appear to be any manufacturers capable of producing competitively priced soda-lime float glass with an upper surface which even begins to rival the stain resistance of the bottom surface which had been in contact with the molten tin.